Automotive vehicles commonly are provided with power-assist systems to facilitate the steering of the vehicle. Typically, servo motors or other operator input sensitive devices are coupled with the steering linkage or the steerable wheels of the vehicle and are operated in response to rotation of the steering wheel shaft.
Electrical power-assist steering units are often employed in vehicles having motors as prime movers such as electric lift trucks, electric passenger vehicles and the like. Such units employ a transducer for monitoring the operator applied torque in the steering shaft and generating an output signal in response thereto. Such a device is disclosed in U.S. Pat. No. 4,173,265 to Kremer which describes a device for measuring the torque in a shaft. The Kremer device employs two rotatable parts of a shaft which are coupled together through an elastic body for the transmission of torque therebetween. A permanent magnet is carried on one of the rotatable parts in rotational alignment with a differential field plate sensor responsive to the position of the magnet for generating an analog output signal representing torque exerted between the parts of the shaft. In the unloaded condition, the magnet lies opposite the center of the differential field plate sensor.
Devices of the type disclosed by Kremer, although having enjoyed some commercial success, suffer from several shortcomings. Typically such transducers are relatively mechanically complex, requiring frequent servicing and having a short useful life. Expense is a corollary to such mechanical complexity. Specifically, certain components such as Hall effect sensors and permanent magnets have become unduly costly in recent years and thus are not well-suited to high-volume, low cost applications.
Another shortcoming common in some prior art devices resides in the fact that electrical sensory elements are disposed on rotating shafts or other components requiring the use of slip rings, wrapped wire umbilicals or the like, which tend to quickly corrode, become intermittent and deteriorate the performance of the transducer. Additionally, many prior art devices exhibit poor sensitivity through an entire range of operation and are therefore limited in accuracy, repeatability, and the ability to provide the vehicle operator with any kind of "feel" for the vehicle.
It will be apparent from a reading of the Specification that the present invention may be advantageously utilized in many different applications, both mobile and fixed, requiring the precise monitoring of applied forces in general and torques in particular. However, the invention is especially useful when applied to an electric power steering system for an electric vehicle and will be described in connection therewith.